Internal combustion engines typically utilize an exhaust gas recirculation (EGR) system to recirculate a controlled portion of exhaust gas generated by the engine into an intake manifold of the engine in order to provide a reduction in NO.sub.x emissions generated by the engine. Typically, a control mechanism is provided which varies the EGR rate according to one or more sensed conditions such as engine temperature, air charge entering the intake manifold of the engine, and engine speed. High EGR rates are preferable to improve fuel economy and reduce emissions. However, the ability to use relatively high EGR rates is hampered by the effects of humidity. Specifically, high humidity reduces the driveability of engines with high EGR operating schedules.
Similarly, humidity has an effect on borderline spark settings which relate to engine knock. Specifically, humidity reduces the tendency of an internal combustion engine to spark knock by lowering the temperature of the end gases in the combustion chamber. The overall effect is an increase in performance as humidity increases. Engine spark control systems are, thus, typically calibrated for worst case humidity levels, i.e., low humidity.
Accordingly, there is a need for a system which provides an optimum EGR rate at a variety of engine operating ranges and atmospheric humidity levels. Additionally, there is a need for an engine control system which provides optimum borderline spark control at a variety of engine operating ranges and atmospheric humidity levels.